THERMAL DOSE OPTIMIZATION VIA TEMPORAL SWITCHING IN ULTRASOUND SURGERY

Temporal switching has been simulated and implemented in vivo experime nts as a method to optimize thermal dose in ultrasound surgery. By opt imizing the thermal dose over a tissue volume, the peak temperature is decreased, less average power is expended, and overall treatment time is shortened. To test this hypothesis, a 16 element, spherically sect ioned array has been constructed for application in ultrasound surgery guided by magnetic resonance imaging. A simulation study for the arra y was performed to determine an optimal treatment from a set of multip le focus fields. These fields were generated using the mode scanning t echnique with power levels determined numerically using a direct weigh ted gradient search in the attempt to create an optimally uniform ther mal dose over a 0.6 X 0.6 X 1.0 cm(3) tissue volume. Comparisons of th e switched fields and a static multiple focus field indicate that the switching technique can lower power requirements and decrease treatmen t time by 20%. More importantly, the peak temperature of the sonicatio n was lowered 13 degrees C, thus decreasing the possibility of cavitat ion. The simulated results of the 16 element array were then experimen tally tested using MRI to noninvasively monitor temperature elevations and predict lesion size in rabbit thigh muscle in vivo. In addition, the results show that the switching technique can be less sensitive to tissue inhomogeneities than static field sonication while creating co ntiguous necrosis regions at equal average powers.